FIG. 1 illustrates a network architecture of an Evolved Universal Mobile Telecommunications System (E-UMTS) as a mobile communication system having the related art and the present invention applied thereto. The E-UMTS system has evolved from the existent UMTS system and its basic standardization is undergoing in 3GPP. Such E-UMTS system may also be referred to as a Long Term Evolution (LTE) system.
E-UMTS network may be divided into E-UTRAN and Core Network (CN). The E-UTRAN includes a terminal (User Equipment, referred to as ‘UE’ hereinafter), a base station (referred to as ‘eNode B’ hereinafter), a Serving Gateway (S-GW) located at the end of the network to be connected to an external network, and a Mobility Management Entity (MME) for managing the mobility of the UE. One or more cells may exist in one eNode B.
FIG. 2 illustrates a radio interface protocol architecture between UE and base station based on the 3GPP radio access network standard. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer and a network layer, and has vertical planes comprising a user plane for transmitting data information and a control plane for transmitting a control signaling. The protocol layers can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an Open System Interconnection (OSI) standard model widely known in communications systems.
Hereinafter, each layer in the radio protocol control plane in FIG. 2 and a radio protocol user plane in FIG. 3 will be described.
A first layer, as a physical layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to its upper layer, called a Medium Access Control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel. Data is transferred via a physical channel between different physical layers, namely, between the physical layer of a transmitting side and the physical layer of a receiving side.
The MAC layer located at the second layer provides a service to an upper layer, called a Radio Link Control (RLC) layer, via a logical channel. The RLC layer of the second layer supports reliable data transmissions. A Packet Data Convergence Protocol (PDCP) layer of the second layer is used to efficiently transmit IP packets, such as IPv4 or IPv6, on a radio interface with a relatively small bandwidth. For this purpose, the PDCP layer reduces the size of an IP packet header which is relatively great in size and includes unnecessary control information, namely, performs a function called header compression. Also, the PDCP layer is used to encode C-plane data, for example, an RRC message. The PDCP layer 3 may also be used to encode U-Plane data.
A Radio Resource Control (RRC) layer located at the lowermost portion of the third layer is only defined in the control plane. The RRC layer controls logical channels, transport channels and physical channels in relation to configuration, re-configuration and release of Radio Bearers (RBs). Here, the RB signifies a service provided by the second layer of the radio protocols for data transmissions between the terminal and the E-UTRAN.
Hereinafter, a random access channel (RACH) will be described. The RACH channel may be used to transmit uplink data with a short length, particularly, used when a terminal without being allocated with a dedicated radio resource has to transmit a signaling message or user data on uplink. Alternatively, the RACH channel may be used by a base station to indicate the performance of an RACH procedure to a terminal.
Hereinafter, a random access channel (RACH) procedure provided in an LTE system will be described. The RACH procedure provided in the LTE system may be divided into a contention based (RACH) procedure and a non-contention based RACH procedure. The division may depend on whether a random access preamble used in the RACH procedure is selected directly by a terminal or by selected a base station.
In the non-contention based RACH procedure, a terminal may use a specific random access preamble directly allocated thereto by a base station. Therefore, when the base station allocates the specific random access preamble only to the terminal, the random access preamble may be used only by the terminal, and thus other terminals may not use it. Hence, an one-to-one (1:1) relation between the random access preamble and the terminal using the random access preamble may exist, whereby it can be said that there is no contention. In this case, the base station can identify a terminal having transmitted a random access preamble as soon as receiving the random access preamble. So, the non-contention based RACH procedure may be efficient from this perspective.
On the other hand, in the contention based RACH procedure, a terminal may randomly select a random access preamble for transmission, among random access preambles. Accordingly, a plurality of terminals may have a chance of always using the same random access preamble. Therefore, even if a base station receives a specific random access preamble, it cannot identify which terminal has transmitted the random access preamble.
In general, a terminal may carry out a random access channel (RACH) procedure in the following cases, namely, 1) when the terminal initially accesses a base station because it has no RRC connection established with the base station, 2) when the terminal initially accesses a target cell during a handover procedure, 3) upon being requested by a command from the base station, 4) when uplink data is generated under the condition that an uplink (time) synchronization is not matched or a designated radio resource which is used to request for a radio resource has not been allocated, and 5) during a restoration procedure at the time of radio link failure or handover failure.
Based upon the above description, FIG. 4 illustrates operations between a terminal and a base station during a contention based random access channel (RACH) procedure.
First, in a contention based random access, a terminal randomly selects one random access preamble from a group of random access preambles indicated by system information or handover command. The terminal then selects a physical random access channel (PRACH) resource capable of transmitting the random access preamble so as to transmit the random access preamble (Step 1). Such preamble is referred to as ‘RACH MSG 1’.
After transmitting the random access preamble, the terminal attempts to receive a response message in response to the random access preamble within a random access response reception window indicated by the system information or handover command (Step 2). In more detail, the random access response information may be transmitted in the format of MAC protocol data unit (PDU). The MAC PDU may be transmitted via a physical downlink shared channel (PDSCH). A physical downlink control channel (PDCCH) may also be transmitted in order for the terminal to appropriately receive information transmitted over the PDSCH. That is, the PDCCH may include information related to a terminal to receive the PDSCH, information related to frequency and time of PDSCH radio resources, a transmission format of the PDSCH, and the like. Here, if the terminal has successfully received the PDCCH coming to the terminal itself, then the terminal properly receives a random access response transmitted over the PDSCH according to the information included in the PDCCH. The random access response may include a random access preamble identifier (ID), a UL grant (uplink radio resource), a temporary cell-radio network temporary identifier (C-RNTI) and a timing advance command (or time alignment command, or time sync compensation value). Since one random access response may include random access response information for one or more terminals, the random access preamble identifier is required to indicate to which terminal the UL grant, the temporary C-RNTI and the timing advance command information are available. The random access preamble identifier matches with the random access preamble selected by the terminal in Step 1.
Here, when the terminal receives a random access response available thereto, then the terminal processes (handles) each of the information included in the random access response. That is, the terminal may apply the timing advance command and stores the temporary C-RNTI. Also, the terminal uses the UL grant to send data stored in its buffer or newly generated data to the base station (Step 3). Here, the base station cannot determine which terminals perform the RACH procedure during the contention based RACH procedure, but should identify such terminals for resolving the contention layer. Hence, the UL grant should essentially include a terminal identifier among other data (hereinafter, referred to as ‘message 3). Here, there are two methods for including the terminal identifier. A first method is configured such that if a terminal has an available cell identifier already allocated in the corresponding cell before the RACH procedure, the terminal transmits its cell identifier via the UL grant, while the terminal transmits its specific identifier (e.g., S-TMSI or random ID) if the terminal has not been allocated with an available cell identifier before the RACH procedure. In general, the specific identifier is longer than the cell identifier. In Step 3, if the terminal has sent data via the UL grant, the terminal initiates a contention resolution timer.
After transmitting data including its identifier via the UL grant included in the random access response, the terminal waits for an instruction from a base station for contention resolution. That is, the terminal attempts to receive the PDCCH for receiving a specific message (Step 4). Here, the PDCCH reception may be performed by two methods. As aforementioned, if the terminal identifier transmitted via the UL grant is the cell identifier, the terminal may attempt the PDCCH reception using its cell identifier. On the other hand, if the identifier is the specific identifier, the terminal may attempt the PDCCH reception using the temporary C-RNTI included in the random access response. Afterwards, for the former, if the terminal has received the PDCCH via its cell identifier before the expiration of the contention resolution timer, then the terminal may determine that the RACH procedure has normally been performed so as to terminate the RACH procedure. For the latter, if the terminal has received the PDCCH via the temporary C-RNTI before the expiration of the contention resolution timer, the terminal may check data (hereinafter, referred to as message 4) sent over the PDSCH instructed by the PDCCH. If its specific identifier is included in contents of the data, then the terminal may determine that the RACH procedure has normally been performed, so as to terminate the RACH procedure. Here, a message or MAC PDU received in Step 4 is usually referred to as an RACH MSG 4.
Hereinafter, description will be given of a method in which a terminal receives downlink data in an LTE system. FIG. 5 is an exemplary view illustrating a radio resource allocation according to the related art.
Physical channels may roughly be divided into two channels in downlink, namely, a physical downlink control channel (PDCCH) and a physical downlink shared channel (PDSCH). The PDCCH has no direct relation to the transmission of user data. The PDCCH is rather used to transmit control information required for the application of other physical channels. Briefly explaining, the PDCCH can be used for the control of other physical channels. In particular, the PDCCH is used by a terminal for the transmission of information required to receive the PDSCH. For example, those Information indicating at which transmission time interval data is sent, which frequency band is used to send data, to which terminal data is sent, what size data has, and the like, may be sent over the PDCCH. Thus, each terminal receives the PDCCH at a specific TTI and determines whether data to be received by the terminal itself is sent. If it is informed that the data to be received by the terminal is sent, the terminal further receives the PDSCH using information related to a frequency or the like indicated in the PDCCH. Information on to which terminal (one or plural terminals) PDSCH data is transmitted, how the terminal(s) should receive and decode the PDSCH data and the like, may be included in the PDCCH for transmission.
For example, it is assumed that radio resource information A (e.g., a frequency location) and transmission format information B (e.g., transmission block size information, modulation and coding information and the like) are CRC-masked to a radio network temporary identifier (RNTI) called C to be transmitted over the PDCCH in a specific sub frame. One terminal or two or more terminals staying in a corresponding cell monitor(s) the PDCCH using the RNTI information belonging to the terminal(s). Under the assumption, a CRC error may not occur in a terminal having the RNTI called C when decoding the PDCCH. Therefore, the terminal may decode the PDSCH for data reception by using the transmission format information B and the radio resource information A. On the other hand, under the assumption, the CRC error may occur in a terminal without the RNTI called C when decoding the PDCCH. Therefore, the terminal may not receive the PDSCH.
During the procedure, in order to inform to which terminals radio resources are allocated, the RNTI is transmitted over each PDCCH. Such RNTIs may include a dedicated RNTI and a common RNTI. The dedicated RNTI may be allocated to one terminal, and used for transmission and reception of data of the corresponding terminal. The dedicated RNTI may be allocated to terminals having information registered in a base station. On the other hand, the common RNTI may be used when terminals, of which information have not been registered in the base station so as not to have the dedicated RNTI allocated thereto, transmit and receive data to/from a base station, or used for the transmission of information, such as system information, commonly applied to a plurality of terminals.
As mentioned above, a base station and a terminal construct the E-UTRAN. Radio resources within one cell may be composed of uplink radio resource and downlink radio resource. The base station manages allocation and control of the uplink and downlink radio resources of the cell. That is, the base station determines which radio resource is to be used for which terminal with time information. For example, the base station may determine to allocate a frequency in the range of 100 MHz to 101 MHz to a user 1 after 3.2 seconds, for a downlink data transmission for 0.2 seconds. After the determination, the base station informs the corresponding terminal of such information, such that the terminal can receive the downlink data. Similarly, the base station may determine when and which terminal is allowed to transmit uplink data using how many and which radio resources. The base station may then inform the corresponding terminal of the determination such that the terminal can transmit the uplink data using the radio resources for the corresponding time.
Unlike the related art, the dynamic management of radio resources by the data station allows the efficient use of radio resources. The related art technique is configured such that one terminal keeps using one radio resource during a call connection. This technique is irrational, considering that many services are recently provided based upon an internet protocol (IP) packet. It is because most packet services do not continuously generate packets during a call connection but contain non-transmission intervals during the call connection. In spite of this, the continuous allocation of radio resources to one terminal is inefficient. To solve the problem, the E-UTRAN system employs the aforesaid method for allocating a radio resource to a terminal, while service data exists, only when the terminal needs the radio resource.
In more detail, in order to efficiently use radio resources in the LTE system, the base station should know what kind of data and how many data each user wants to send. For downlink data, it is transferred from an access gateway to the base station. The base station thus knows how many downlink data should be transferred to each user. On the other hand, for uplink data, if a terminal does not inform the base station of information related to uplink data that it wants to send, the base station cannot know how many uplink data each terminal needs. Hence, in order for the base station to appropriately allocate uplink radio resources to terminals, each terminal should provide the base station with information required for scheduling radio resources.
To this end, a terminal informs to the base station if it has data to send, and the base station sends a radio resource allocation message to the terminal based upon the information.
At the process, namely, when the terminal informs the base station that it has data to send, the terminal informs the base station of the amount of data stored in its buffer, which is called as a buffer status report (BSR).
However, the BSR is generated in the format of a MAC control element (MAC CE) and included in a MAC PDU to be transmitted from the terminal to the base station. That is, an uplink radio resource is required for the BSR transmission, which means that uplink radio resource allocation request information for the BSR transmission should be sent. When the BSR is generated, if there is an uplink radio resource allocated, the terminal immediately sends the BSR using the uplink radio resource. However, when the BSR is generated, if there is no uplink radio resource allocated, the terminal performs a scheduling request (SR) procedure (i.e., resource allocation request procedure).
The SR procedure may be divided into two ways, namely, a method using a dedicated scheduling request (D-SR) channel set for a physical uplink control channel (PUCCH) and a method using a RACH procedure. That is, once the SR procedure is triggered, if the terminal has an allocated D-SR channel, then the terminal uses the D-SR channel to send a radio resource allocation request. If the terminal does not have the D-SR channel allocated thereto, then the terminal starts the RACH procedure. In case of using the D-SR channel, the terminal sends a radio resource request allocation signal on uplink via the D-SR channel.
The SR procedure may be continuously performed until the terminal is allocated with a UL-SCH resource.
During the procedure, the BSR sent by the terminal is used to inform the amount of buffer for each logical channel group (LCG), other than sending information related to the amount of buffer for each logical channel. That is, the base station calculates the amount of buffer for each designated group. Maximum four LCGs are defined for one terminal. During the procedure, there are two types of BSR, including long buffer status report (long BSR) and a short buffer status report (short BSR). The long BSR includes information related to the amount of buffer for all of the four LCGs, while the short BSR includes information related to the amount of buffer for one LCG.
The related art radio resource allocation request has the following problem. In general, a plurality of terminals exists in one cell and a base station firstly allocates radio resources to a terminal with a high priority and a channel with a high priority. Therefore, depending on cases, in spite of receiving a radio resource allocation request message or BSR from a specific terminal, the base station may not allocate a radio resource to the terminal. In this case, if the terminal keeps sending the radio resource allocation request, it may cause the consumption of uplink radio resources, which results in an interference with radio resources. In addition, in the above case, while a terminal performs the RACH procedure, the base station may directly allocate a radio resource to the terminal. For example, the terminal may be allocated with a radio resource from the base station using its dedicated identifier after completing Step 2 of the RACH procedure. However, the second message of the RACH procedure includes information related to the allocation of radio resource to a terminal having performed the RACH procedure. In this case, if the terminal continues to perform the RACH procedure, the radio resource allocated using the dedicated identifier is consumed. Also, the D-SR channel is effective only when the terminal has an uplink synchronization. That is, if the terminal does not have the uplink synchronization, even if the terminal uses the D-SR channel, the base station cannot appropriately receive the radio resource allocation request from the terminal. In this case, if the terminal keeps sending the radio resource allocation request without considering such situation, it may only cause the interference with radio resources.